John Gill Commentary Genesis 48:18

"And Joseph said unto his father, Not so, my father; for this is the first-born; put thy right hand upon his head." — Genesis 48:18 (ASV)

And Joseph said to his father, not so, my father
It is not right, it should not so be, that the right hand should be put on the youngest, and the left hand on the eldest; for this [is] the firstborn ;
directing him to Manasseh, and seeking to guide his hand towards him: put your right hand upon his head ;
Joseph was for proceeding according to the order of birthright, but Jacob was directed by a spirit of prophecy, as follows.